1. Field of the Invention
The present invention relates to a telephone and more particularly to the telephone having a separate power circuit for supplying power to a speech network.
2. Description of the Related Art
There has been known a conventional telephone, as shown in FIG. 1, connected to a telephone line 37. The telephone is connected to remote telephones through a network of exchanges. The conventional telephone includes a speech network 31, a handset 32, a dialer circuit 34, a dial 33, a ringer circuit 36, and a bell 35. The speech network 31 transmits voice signals to, and receives voice signals from, the telephone line 37. The handset 32 includes a speaker 39 and a microphone 38. The speaker 39 produces voice messages according to voice signals output from the speech network 31. The microphone 38 converts a voice into voice signals and inputs them to the speech network 31. The dialer circuit 34 converts input from the dial 33 into signals for dialing remote telephone. The ringer circuit 36 turns the bell 35 ON when a 16 Hz call signal is received from a telephone exchange through the telephone line 37.
Conventional telephones are driven by the current supplied from the telephone exchange via the telephone line 37, thus each circuit of the telephone, such as the speech network 31, the dialer circuit 34, and the ringer circuit 36, is directly connected to the telephone line. Because the current supplied from the telephone exchange fluctuates caused by changing telephone line connections, a special-purpose IC is used for the speech network 31 so as to be operable even under such an unstable condition. The characteristics of the speech network IC are determined by resistors and capacitors contained therein.
Resistors and capacitors used in the speech network in association with the speech network IC are recommended so as to meet the characteristics of the standard bell 35, dial 33 and handset 32.
However, in view of the fact that acoustic performances of individual handsets differ depending on the external shape of the handset 32 and characteristics of the microphone 38, the speaker 39 and the like. Therefore, for such telephones that employ nonstandard handset, satisfactory conversation conditions are not obtainable with the circuit of the speech network 31 configured by the recommended components.
Moreover, the use of the special-purpose speech network IC makes it difficult to configure the speech network circuit so as to have adequate characteristics of incoming transmission, outgoing transmission, and sidetone (the sound of the speaker's own voice as heard in his telephone receiver) and also impedance characteristic required for the speech network circuit under the fluctuating power supply condition. Consequently, the configuration of the speech network circuit has been difficult.
Using the special-purpose speech network IC in the speech network 31 has hindered attempts to reduce the cost of conventional telephones. Attempts have been made to use less expensive, general-purpose IC in its place, but these general-purpose ICs are unable to cope with fluctuations in current supply and the circuit configuration becomes far more difficult with the general-purpose IC.